A diesel engine is equipped with an exhaust gas processing device including a diesel oxidation catalyst (DOC) disposed in an exhaust channel and a diesel particulate filter (DPF) disposed on the downstream side of the DOC. A diesel particulate filter (DPF) is a device for collecting particulate matters (PM) contained in exhaust gas discharged from the diesel engine. A DPF is normally formed of ceramic or the like in a honeycomb-shaped monolith with adjacent vents closed alternately on an inlet side and an outlet side so that exhaust gas passes through filtering walls, which remove PM. A DPF may support a catalyst. Accumulation of PM in a DPF eventually brings about clogging, which leads to not only a decrease in the PM-collecting performance of the DPF but also to an increase in an exhaust pressure and thus has a negative effect on fuel efficiency. Thus, it is necessary to perform forced regeneration to remove PM accumulated on the DPF every time when a PM-accumulation amount reaches a predetermined amount or when an engine operation time reaches a predetermined period.
Forced regeneration of a DPF is performed by forcedly increasing the inlet temperature of the DPF. In general, the inlet temperature of a DPF is forcedly increased by supplying non-combusted fuel to an exhaust-gas processing device by post injection, which is to inject fuel after a main-combustion injection timing, and oxidizing the non-combusted fuel with a diesel oxidization catalyst (DOC) so that the temperature of the non-combusted fuel increases. Further, combustion may be supplied by exhaust-duct injection to an exhaust channel on the downstream side of the engine. A DOC is normally formed of ceramic or the like in a honeycomb-shaped monolith, similarly to the above described DPF, and supports an oxidization catalyst on the inner surface of the DOC.
In such an exhaust-gas post treatment device, if a state with a low operation load and a low exhaust-gas temperature continues, SOF or soot of non-combusted fuel or the like adheres to an upstream end surface of the DOC, and clogging of the DOC gradually progresses. If the DOC gets clogged, the exhaust pressure increases and the fuel efficiency decreases, and the non-combusted fuel is not sufficiently oxidized by the DOC in the forced regeneration and slips. In response, more and more non-combusted fuel is supplied to the DOC to increase the inlet temperature of the DPF to a predetermined temperature, thereby deteriorating fuel efficiency. Further, the slipped fuel is oxidized and heated by the DPF supporting a catalyst, and thus promotes abnormal combustion of the PM, thereby raising a risk of heat damage to the DPF. Further, if the temperature is increased by late post injection, the post injection amount increases and a risk of oil dilution increases in accordance.
To prevent such clogging of the DOC, Patent Document 1 discloses a technique, with an exhaust-gas purification device for a diesel engine, to calculate an adhering amount of non-combusted fuel adhering to the surface of the DOC, reduce the opening degree of an intake throttle if the calculated adhering amount of the non-combusted fuel is greater than a predetermined value to increase the exhaust-gas temperature by decreasing the air excess ratio, and promote oxidization removal of HC adhering to the surface of the DOC.
Further, Patent Document 2 discloses a technique to combust and remove non-combusted fuel adhering to a surface of a DOC by maintaining an exhaust-gas temperature at a predetermined temperature with an exhaust-gas temperature maintaining unit after completion of a regeneration process of a DPF in a diesel engine.